Add opt-out for rate limit model nudge (#6433)

## Summary
- add a `hide_rate_limit_model_nudge` notice flag plus config edit
plumbing so the rate limit reminder preference is persisted and
documented
- extend the chat widget prompt with a "never show again" option, and
wire new app events so selecting it hides future nudges immediately and
writes the config
- add unit coverage and refresh the snapshot for the three-option prompt

## Testing
- `just fmt`
- `just fix -p codex-tui`
- `just fix -p codex-core`
- `cargo test -p codex-tui`
- `cargo test -p codex-core` *(fails at
`exec::tests::kill_child_process_group_kills_grandchildren_on_timeout`:
grandchild process still alive)*

------
[Codex
Task](https://chatgpt.com/codex/tasks/task_i_6910d7f407748321b2661fc355416994)

codex-rs/tui/src/chatwidget.rs

@@ -522,6 +522,7 @@ impl ChatWidget {
                     .unwrap_or(false);
 
             if high_usage
+                && !self.rate_limit_switch_prompt_hidden()
                 && self.config.model != NUDGE_MODEL_SLUG
                 && !matches!(
                     self.rate_limit_switch_prompt,
@@ -1710,7 +1711,18 @@ impl ChatWidget {
             .find(|preset| preset.model == NUDGE_MODEL_SLUG)
     }
 
+    fn rate_limit_switch_prompt_hidden(&self) -> bool {
+        self.config
+            .notices
+            .hide_rate_limit_model_nudge
+            .unwrap_or(false)
+    }
+
     fn maybe_show_pending_rate_limit_prompt(&mut self) {
+        if self.rate_limit_switch_prompt_hidden() {
+            self.rate_limit_switch_prompt = RateLimitSwitchPromptState::Idle;
+            return;
+        }
         if !matches!(
             self.rate_limit_switch_prompt,
             RateLimitSwitchPromptState::Pending
@@ -1744,6 +1756,10 @@ impl ChatWidget {
         })];
 
         let keep_actions: Vec<SelectionAction> = Vec::new();
+        let never_actions: Vec<SelectionAction> = vec![Box::new(|tx| {
+            tx.send(AppEvent::UpdateRateLimitSwitchPromptHidden(true));
+            tx.send(AppEvent::PersistRateLimitSwitchPromptHidden);
+        })];
         let description = if preset.description.is_empty() {
             Some("Uses fewer credits for upcoming turns.".to_string())
         } else {
@@ -1769,6 +1785,17 @@ impl ChatWidget {
                 dismiss_on_select: true,
                 ..Default::default()
             },
+            SelectionItem {
+                name: "Keep current model (never show again)".to_string(),
+                description: Some(
+                    "Hide future rate limit reminders about switching models.".to_string(),
+                ),
+                selected_description: None,
+                is_current: false,
+                actions: never_actions,
+                dismiss_on_select: true,
+                ..Default::default()
+            },
         ];
 
         self.bottom_pane.show_selection_view(SelectionViewParams {
@@ -2386,6 +2413,13 @@ impl ChatWidget {
         self.config.notices.hide_world_writable_warning = Some(acknowledged);
     }
 
+    pub(crate) fn set_rate_limit_switch_prompt_hidden(&mut self, hidden: bool) {
+        self.config.notices.hide_rate_limit_model_nudge = Some(hidden);
+        if hidden {
+            self.rate_limit_switch_prompt = RateLimitSwitchPromptState::Idle;
+        }
+    }
+
     #[cfg_attr(not(target_os = "windows"), allow(dead_code))]
     pub(crate) fn world_writable_warning_hidden(&self) -> bool {
         self.config